could someone explain DFCO and coasting to me? - Page 3 - Fuelly Forums

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Old 04-20-2009, 02:11 PM   #21
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Understand pumping losses.

The more you pump the more you loose.

Opening the throttle increases the "effective" compression of your engine.

A perfect vacuum is O. Atmospheric pressure is 14.7 Psi. No engine creates a perfect vacuum.

Now if you have maximum compression, in most modern engines that 180-200 PSI. On the top of a 4 inch diameter piston that is 200X4X3.1416 PSI.
A perfect vacuum (which doesnt exist) would be -14.7X4X3.1416 PSI

Most modern engines only produce about 22 inches of vacuum. When you let off the throttle a bypass keeps the vacuum lower than a perfect vacuum, because if you had a perfect vacuum and fuel was being injected, it would not combust and you would dump the raw fuel into your catalytic converter.

DFCO eliminates the problem by eliminting the fuel. I dont know how you can not have DFCO in some form or the other, but I guess it's possible.

The pumping losses would be higher if you opened the throttle because you would have more compression. This is easily understood if you realize you must open the throttle when doing a compression test, in order to get the proper reading.

When you open the throttle during a compression test, the craking speed of the engine drops off, becasue the engine is doing more work creating higher compression. A compression test is the same as decelerating without fuel, just at higher revolutions.

If your compression ratio is 10 to 1 the pressure on the top of the piston is 10times greater than could ever be created by even a perfect vacuum. In fact it is almost 15 times greater becasue you do no have a perfect vacuum at idle.

Pumping losses consist of all energy losses relative to moving air through an engine. The actual individual losses are numerous, each contributing some percentage of total pumping losses. I think I can name something like 20 differect individual events that are all factors, and their percentages change relative to engine speed.

regards
gary
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Old 04-20-2009, 02:15 PM   #22
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One example. Many years ago Ford discovered that they could reduce the oil pressure in their new engines by simply making the pressure relief valve weaker. Any more than 35 PSI or pressure was a waste of energy, and they found that they could get an average of .5 MPG better mileage by just reducing the strength of the spring in the oil pressure relief valve.

Modern lower viscosity oils also reduce the energy needed to push oil through the engine, a fact we all know.

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Old 04-20-2009, 02:16 PM   #23
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Interesting. I thought that, when not burning fuel, air getting compressed gives most of the energy back on the power stroke, effectively acting as a spring; and I thought that all pumping losses are at the throttle, intake mainfold, valves, and exhaust.

I would think you need to open the throttle during a compression test to make sure pumping losses don't prevent the cylinder from filling properly.
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Old 04-20-2009, 02:18 PM   #24
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If your engine has port fuel injection, you can actually hear DFCO. Take a screwdriver and place it on the injector. Put the handle up to your ear and listen for the ticking as the injector cycles.

Rev up the engine and listen to the cycling frequency increase. Then let off the throttle and see if the ticking of the injector stops altogether.

Thats DFCO

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Old 04-20-2009, 02:29 PM   #25
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It does to a point Holy Cow, but consider this. When the manufacturers started using cylinder deactivation, they found they only increased mileage by 10 %.

Since the rotational inertai of the engine is already pulling the piston down, and the reciprocating masses are fighting the rotating masses every inch of the way, the benefit of the "air spring" effect is relatively small.

Thats my opinion, but I think the evidence supports it.

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Old 04-20-2009, 02:40 PM   #26
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Your point is valid HC, I think you are to smart to make any invalid points.

Without combustion the same air spring effect applies to pushing the piston down, during the normal power stroke.

In fact the only power produced by an engine is during that phase of the cycle, but you know that already.

Power during combustion is produced by the differential pressure due to combustion. The overall ratio is about 7 to one, the comparison of the kelvin temperature of the compressed charge to the kelvin temperature of the ignited mixture.

When you create a vacuum, the temperature of the air is reduced, so the air spring effect of that same vacuum on pulling the piston up is reduced as well.

When compressed the heat content of the compressed air is increased, which would increase its effective pressure somewhat. This may help its push on the piston on the downstroke, but it also hurts on the upstroke.

This can get complicated easily .

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Old 04-20-2009, 03:00 PM   #27
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LOL, I am definitely not too smart to make invalid points. I do it all the time.

I only cited the air spring effect as minimizing the energy losses involved in compressing more air; I didn't mean to imply that you'd get any net energy increase out of it during DFCO. If I could make that work I'd invent a perpetual motion machine and give you a cut of the profits!
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Old 04-20-2009, 03:01 PM   #28
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A couple of points.

Not many of us here have 4" bores. It's more like 3" for the majority of vehicles on here so the calculation is 200*2.25*pi.

Secondly, you have to press the throttle when doing a compression test because the throttle plate being closed creates a vacuum which lowers the amount of air in your cylinders and thus your readings.

Also, raw fuel in the cat wasn't why they started DFCO or the bypass. An EFI vehicle can meter fuel however low it wants to so dumping raw fuel into the cylinders is a non-issue. Only a carbureted vehicle would need DFCO and it obviously can't get it. .

The bypass is used on a few different vehicles for different reasons. Toyota, for example, uses it to reduce afterfiring during engine braking. You can have a perfect stoich mixture but at high manifold vacuums exhaust gets sucked back into the cylinder during valve overlap and it dilutes the mixture to a point that it won't reliably fire without excessive amounts of oxygen.
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Old 04-20-2009, 03:06 PM   #29
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Quote:
Originally Posted by R.I.D.E. View Post
When you create a vacuum, the temperature of the air is reduced, so the air spring effect of that same vacuum on pulling the piston up is reduced as well.
As temperature decreases density increases. This would increase the vacuum, not decrease it.
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Old 04-20-2009, 03:30 PM   #30
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I would think there would be a too much vacuum with it only being able to pull the air thought a half inch to three quarter inch line. This is what caused my earlier question.
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